This investigation is concerned with ascertaining the sequence of development of fast and slow muscle in crustaceans, particularly the lobster Homarus americanus. Studies will focus on myogenesis in pre and early post larval stages, particularly in regard to a comparison of fast and slow muscle. The primary preparations will be the closer muscles of the large claws and the abdominal muscles. In the abdominal muscles, fast and slow fibers are in anatomically distinct bundles. The former preparation has the advantage that the two closer muscles are composed of all fast muscle fibers in the early post-larval stages, but latter develop asymmetrically into the slow crusher claw closer and the mixed (fast and slow) cutter claw closer muscle. The direction of development is apparently not genetically fixed, so the effects of various treatments such as immobilization, tentotomy and axonotomy will be followed. Parallel studies will be performed on regenerating claws to establish the similarities and differences in this developing system as compared with normal development. The closer muscles, as well as some of the other skeletal muscles exhibit striking growth from the post larval stages to the adult. This growth is at least several thousand-fold and cannot be explained by an increase in fiber diameter. The addition of new fibers might be via myogenesis from myoblasts or from fiber growth and subsequent splitting. There is evidence that the latter is the mechanism utilized but it is not known whether nuclei also divide, maintaining an approximately constant ratio of nuclei to muscle fibers or whether the number of nuclei remains constant, and there are fewer per fiber in larger animals. In crustacean skeletal muscles with two motor axons, one usually behaves like a "fast axon" and the other like a "slow axon", evoking respectively, a twitch and a slow strong contraction. It is unclear whether this is the case in the adult crusher claw, where there is exclusively intermediate and slow muscle. This will be studied in the adult and hopefully through earlier stages in the life history to determine the function of the fast axon in the crusher claw.